1. Field of the Invention
The present invention relates to an optical disk device such as a DVD and a CD, and more particularly concerns a control system which can cancel adverse effects caused by a defect located on the optical disk.
2. Description of the Background Art
In optical disk apparatuses, various methods for reproducing data from a disk in spite of defects such as scratches and stain (hereinafter, referred to as “defective disks”) located thereon have been proposed, and for example, Japanese Patent Application Laid-Open No. 11-259871(1999) discloses a method in which upon reproducing data from a defective disk, the data is reproduced while interpolating error signals.
FIG. 13 is a block diagram that schematically shows a construction (first construction) of a conventional defect compensating device for defective disks in accordance with the above-mentioned publication.
As illustrated in the Figure, a laser light beam, outputted from a light-emitting optical system 15 including a semiconductor LD (Laser diode) at the time of data recording or data reproduction, is converged onto a (DVD) disk 1 through a half mirror 4 and an objective lens 2. Light reflected from the disk 1 at the time of data reproduction is, on the other hand, inputted to a photoelectric transfer element 5 through the half mirror 4.
An actuator driving coil 3 is rigidly connected (firmly connected so as to move integrally) to the objective lens 2, and the driving coil 3 is placed in a magnetic circuit so that the objective lens 2 is shifted by the driving coil 3 in a direction perpendicular to the disk 1.
Based upon the photoelectric transfer signal obtained from the photoelectric transfer element 5, a defect detection signal generation means 14 generates a detect detection signal SD indicating a detection state of the presence or absence of a disk defective area which lacks optical information due to scratches or strain formed on the surface of a disk 1.
Based upon the photoelectric transfer signal obtained from the photoelectric transfer element 5, a detect detection signal generation means 14 generates a detect detection signal SD indicating a detection state of the presence or absence of a disk defective area which lacks optical information due to scratches or stain formed on the surface of a disk 1.
Based upon the defect detection signal SD, the selection switch 19 inputs one of the error signal S6 and the disk physical strain correction signal S16 to a phase compensation means 18. Based upon the inputted signal through the selection switch 19, the phase compensation means 18 outputs an actuator control signal S20 (phase compensation signal S18) to a driver amplifier 10. Based upon the actuator control signal S20, the driver amplifier 10 controls the driving coil 3.
This defect compensation device has an arrangement in which the disk physical strain correction signal generation means 16 for correcting a physical strain inherent to the disk such as eccentricity and vertical deviation is added to a generally-used control loop. In other words, functional blocks, numbers 1 to 6 and 10 and 18, have general control-loop constructions to which functional blocks 14 and 16 for achieving a defective disk reproducing process are added.
Based upon the defect detection signal SD, while detecting a disk error, the selection switch 19 switches the input signal from the phase compensation means 18 from the error signal S6 to the disk physical strain correction signal S16 so that the control loop is cut off. The disk physical strain correction signal generation means 16 outputs the average value of the error signal S6 in a non-defective area as the disk physical strain correction signal S16.
An explanation will be given of a generally-used focusing control by the defect compensation device having the above-mentioned arrangement. In order to reproduce information recorded in an information recording layer on a disk 1, a laser light beam outputted from the light-emitting optical system 15 is always converged on the information recording layer on the disk 1 by the objective lens 2. In order to realize this, the objective lens 2 needs to be position-controlled so as to be always maintained at a predetermined relative position with respect to the disk 1.
The disk 1 has warping, and the absolute amount of the warping is standardized to, for example, not more than ±300 μm in the DVD standard. Since the disk 1 is rotated, the warping of the disk 1 causes the disk 1 to move up and down (hereinafter, referred to as “vertical deviation”); therefore, it is essential to provide tracking control for the objective lens 2. In this case, the object to be controlled is the objective lens 2, the target for the tracking operation is an information recording layer of the disk 1, and the kind of control is a relative positional control between the disk 1 and the objective lens 2. The position-controlling process for the objective lens 2 is provided by feeding a signal formed based upon a relative position error signal between the objective lens 2 and the disk 1 back to the driving means of the objective lens 2.
The above-mentioned construction is achieved by the following means: a means for detecting a relative position with respect to the disk 1 (constituent sections 1, 2, 4 and 5), a means for generating an error (hereinafter, referred to simply as “FES” (Focus Error Signal) between a detected relative position and a target predetermined relative position (error detection means 6), a phase compensation means for stabilizing a position control loop (phase compensation mean 18), and a driving means for changing the position of the objective lens 2 that is an object to be controlled (driving coil 3). Here, the phase compensation means 18 is generally constituted by a phase-upgrading filter for allowing the phase in the vicinity of 1 KHz to advance. The driving coil 3 is rigidly connected to the objective lens 2 so that the objective lens 2 is shifted in a direction perpendicular to the surface of the disk 1 by applying a driving current to the driving coil 3. An actuator control signal (focusing control signal) S20, which is an output of the phase compensation means 18, controls the driving current of the driving coil 3 that is applied by a driver amplifier 10 so that the control system for converging the laser light beam onto an information recording layer of the disk 1 is achieved.
The tracking control is only different from the focusing control in that in addition to the above-mentioned operations, a controlling process for shifting the objective lens 2 in a horizontal direction with respect to the surface of the disk 1 so as to track a track formed on the information recording layer of the disk 1, and is achieved by a construction including the above-mentioned focusing control system; therefore, the description thereof is omitted, and in FIG. 13, the corresponding description is given without distinction between the focusing and tracking controls. Moreover, the following description generally discuss them without the distinction.
Next, an explanation will be given of a defect compensation method by the conventional defect compensation device shown in FIG. 13. The signal to be applied to the driving coil 3 at the time of detecting a defect is defined by the actuator control signal S20 obtained by allowing the disk physical strain correction signal S16 to pass through the phase compensation means 18. Selection is made between the error signal S6 and the disk physical strain correction signal S11 by the selection switch 19 so that state transitions between a normal controlling state and a defect compensating state is carried out.
The disk physical strain correction signal generation means 61 outputs the average value of an error signal in a normal state free from a defect as a disk physical strain correction signal S16; therefore, it is possible to properly maintain the continuity of the error signal S6 even in the event of a defect, and consequently to realize a defect compensation.
The essential condition required for a stable, positive defect compensation process is defined by whether or not a drawing process for the control loop can be normally performed immediately after the defect compensation operation, that is, at the time of completion of the defect (hereinafter, referred to as “defect end”). The conditions for stably drawing the control loop are that the positional difference between the tracking target position at the time of the drawing operation and the position of the objective lens 2 is set in the vicinity of zero and that the relative velocity between the objective lens 2 and the tracking target is close to zero. In other words, if the following conditions 1 and 2 are satisfied; then it is possible to achieve a stable, positive defect compensation process.
Condition 1: The control error (difference between the position of the objective lens 2 and the tracking target position) is zero at the defect end.
Condition 2: The relative velocity (hereinafter, referred to as “relative velocity after the defect compensation process”) between the objective lens 2 and the disk 1 is zero at the defect end.
FIGS. 14A and 14B are explanatory drawings that show defect compensation operation by the defect compensation device. Referring to FIGS. 14A and 14B, the following description will discuss problems with the conventional defect compensation operation. From top to bottom in the Figures, time-wise fluctuations of an all addition signal of the photoelectric transfer signal (hereinafter, referred to as “RF signal”) obtained from respective areas of the photoelectric transfer element 5, the defect detection signal SD, the error signal S6 and the actuator control signal S20 are shown, and FIG. 14A shows a case in which no defect process is carried out, and FIG. 14B shows a case in which the conventional defect process (first method) by using the construction of FIG. 13 is applied thereto. Here, in FIG. 14B, a normal reproducing process is carried out while the defect detection signal SD goes “low” (normal period), and a defect process is carried out while it goes “high” (defect detecting period).
As illustrated in FIG. 14A, in the case of no defect process, from the start of a defect (a point of time from which the RF signal starts to decrease: hereinafter, referred to simply as “defect start”), a disturbance error is mixed into the error signal S6 as the RF signal decreases, and at the time when the RF signal becomes zero (that is, the quantity of reflected light from the disk 1 becomes virtually zero), the error signal S6 itself becomes undetectable (meaningless). Since the control system performs the controlling operation based upon the error signal S6, it tracks the disturbance error. At the defect end, since the control system is taken too far by the disturbance error, a great control error tends to occur, and in the worst case, it sometimes exceeds the detection range (preliminarily determined) of the control error, resulting in an inoperable state in the control.
As illustrated in FIG. 14B, in the case of a conventional defect compensation, when the defect detection signal goes “high”, the disk physical strain correction signal S16 (a representative value (average value) of the error signal S6 at the time of normal playback) is used in place of the error signal S6 so as to make an interpolation process, it is possible to reduce the effects of the disturbance error; however, the following problems tends to arise.
In the case of the general defect detection method, in order to ensure stability in the normal reproducing mode, setting is made so as to detect the presence of a defect when the quantity of reflected light from the disk becomes not more than a predetermined value that is slightly lower than the normal peak value, that is, a predetermined non-sensitive band is provided. With respect to the construction of such a typical defect detection signal generation means 14, as illustrated in FIGS. 14A and 14B, a construction has been proposed in which, at the time t1 when a signal obtained by peak-detecting the RF signal becomes not less than a value V1 that is lower than the normal peak value by ΔV, the defect detection signal SD is allowed to go “high”, thereby entering the defect detecting state.
In this construction, during the time ΔT from the defect start until the RF signal has becomes lower than the value V1, the defect detecting process becomes inoperable; therefore, there might be an inevitable time lag in the defect detection with respect to a true defect. Even during the inevitable time lag period ΔT, since the signal is optically being influenced by the defect, a disturbance error tends to be mixed into the error signal S6.
In the conventional defect compensation method (first method) shown in FIG. 14B, it is not possible to carry out an interpolation process on the disturbance error during the inevitable time lag period ΔT. In general, the residual disturbance error during the time lag period ΔT has frequency components of several hundreds to several kHz, and this is further emphasized by the phase compensation means 18 at the time t2 when the selection switch 19 makes a switch from the disk physical strain correction signal S16 to the error signal S6 upon receipt of the trailing edge “L” of the defect detection signal SD, with the result that the actuator control signal S20 is taken too far, causing an inevitable control deviation at the defect end.
In other words, the conventional defect compensation device shown in FIG. 13 fails to satisfy both of the aforementioned conditions 1 and 2 required for a stable, positive defect compensation process.
In order to reduce the influence of the emphasis given on the disturbance error contained in the error signal S6 by the phase compensation means 18, for example, an arrangement has proposed in which the disk physical strain correction signal generation means 16 and the selection switch 19 are placed on stages after the phase compensation means 18.
FIG. 15 is a block diagram that shows a second construction of the conventional defect compensation device. As illustrated in this Figure, the phase compensation means 18 receives the error signal S6, the disk physical strain correction signal generation means 16 outputs the disk physical strain correction signal S16 based upon the phase compensation signal S18, and the selection switch 19 sends either of the phase compensation signal S18 and the disk physical strain correction signal S16 to the driver amplifier 10 as the actuator control signal S20. Here, since the other arrangements are the same as those shown in FIG. 13, the description thereof is omitted.
FIGS. 16A and 16B are explanatory drawings that show a defect compensation operation by the defect compensation device shown in FIG. 15. In these Figures, the definitions on the respective waveforms are the same as those of FIGS. 14A and 14B, and FIG. 16A shows a case in which no defect process is carried out, and FIG. 16B shows a case in which the conventional defect process (the second method) is carried out by the construction shown in FIG. 15.
A pulse (hereinafter, referred to as “disturbance pulse”), which is caused by a disturbance error in the actuator control signal S20, and shown on lower right of FIG. 16B, is improved as compared with the first method, since the actuator control signal S20 is switched to the disk physical strain correction signal S16 by the selection switch 19 during the defect detection period while the defect detection signal SD is going “high”; however, the actuator control signal S20 is always subjected to a certain amount of a residual disturbance pulse occurring during the time lag period ΔT before the time t1. The actuator control signal S20 is kicked by the residual disturbance pulse, with the result that a control deviation occurs from a predetermined velocity at the defect end. Consequently, even in the construction shown in FIG. 15, it is not possible to satisfy both of the conditions 1 and 2 required for a stable, positive defect compensation operation.
FIGS. 17A to 17C are explanatory drawings that show influences caused by the application of the disturbance pulse on the velocity and position of the actuator. Referring to FIGS. 17A to 17C, the following description will generally discuss the above-mentioned problems. Supposing that the mechanical characteristics of the actuator including the driving coil 3 that is a subject to be controlled are secondary systems, the position, velocity and acceleration (in proportion to the signal applied to the driving coil 3) of the objective lens 2 are defined as shown by Expressions (I) to (III).
[Expression 1]Position=X (t)  (I)[Expression 2]Velocity={dot over (X)} (t)  (II)[Expression 3]Acceleration={umlaut over (X)} (t)  (III)
In FIGS. 17A to 17C, FIG. 17A represents the acceleration, FIG. 17B represents the velocity, and FIG. 17C represents the positional change with time. Assuming that the disturbance pulse has a rectangular waveform as shown in FIG. 17A, the influences this rectangular waveform exerts on the velocity and position of the actuator are explained as follows:                (1) During the application of the disturbance pulse, the velocity of the actuator increases in a manner of a linear function, and when the application of the disturbance pulse is stopped, the velocity at the time of the end of the disturbance pulse is maintained (see FIG. 17B).        (2) During the application of the disturbance pulse, the position of the actuator increases in a manner of a quadratic function, and even when the application of the disturbance pulse is stopped, the position keeps increasing linearly while maintaining the gradient at the time of the end of the disturbance pulse (see FIG. 17C).        
The above-mentioned facts indicate that due to (1), it is not possible to satisfy the condition 2 required for a stable, positive defect compensation operation (that is, the relative velocity after the defect compensation process is zero), and that due to (2), it is not possible to satisfy the condition 1 (that is, the control error is zero). Since the timing of the disturbance pulse end coincides with the start of the defect detection period, the conventional defect compensation method makes the positional error greater in proportion to the defect detection period, sometimes resulting in a case in which the error detection range is exceeded.